How can I make metal look decayed and rotted
Final:



Overview:

Basically you can use the same method as for creating any other rust material with one exception at the end - mix your rust material with Transparent shader trough "rust mask", created beforehand. This will create hard holes in your mesh. I've used Greater Than math node to harden texture.




  There is downside to this method. You can't add thickness to the mesh.
  If you add Solidify modifier, the edges will be empty. You can try to
  work around this with Volume texture but it could be an overkill.